1. Field of the Invention
The present invention relates to an audio-data encoding apparatus, an audio-data encoding method, an audio-data decoding apparatus, and an audio-data decoding method, each of which achieves scalability with respect to lossy compression and lossless compression.
2. Description of the Related Art
An audio-data encoding apparatuses has been proposed, which performs lossy compression on an input audio signal to generate a core stream, performs lossless compression on a residual signal to generate an enhanced stream, and combines these streams to achieve scalability with respect to the lossy compression and the lossless compression (see Patent Document 1: U.S. Patent Appln. Publication No. 2003/0171919). An audio-data decoding apparatus can decode a core stream to generate a lossy decoded audio signal, and can decode the core stream and an enhanced stream, and adds these decoded streams to generate a lossless decoded audio signal.
FIG. 1 schematically shows an example of the configuration of such an audio-data encoding apparatus used in the past. As shown in FIG. 1, the audio-data encoding apparatus 100 includes a lossy-core encoder unit 101, a lossy-core decoder unit 102, a delay-correcting unit 103, a subtracter 104, a lossless-enhance encoder unit 105, and a stream-combining unit 106.
In the audio-data encoding apparatus 100, the lossy-core encoder unit 101 performs lossy compression on an input audio signal that is a pulse-code modulation (PCM) signal to generate a core stream. The lossy-core decoder unit 102 decodes the core stream, to generate a lossy decoded audio signal. The delay-correcting unit 103 delays the input audio signal by the time the input audio signal has been delayed in the lossy-core encoder unit 101 and lossy-core decoder unit 102. The subtracter 104 subtracts the lossy decoded audio signal from the input audio signal delayed by the delay-correcting unit 103, thus generating a residual signal. The lossless-enhance encoder unit 105 performs lossless compression on the residual signal to generate an enhanced stream. The stream-combining unit 106 combines the core stream and the enhanced stream to generate a scalable lossless stream.
FIG. 2 schematically shows the configuration of an audio-data decoding apparatus 110 that is designed for use in combination with the audio-data encoding apparatus 100 described above. As shown in FIG. 2, the audio-data decoding apparatus 110 includes a stream-dividing unit 111, a lossy-core decoder unit 112, a lossless-enhance decoder unit 113, and an adder 114.
In the audio-data decoding apparatus 110, the stream-dividing unit 111 divides the input scalable lossless stream into a core stream and an enhanced stream. The lossy-core decoder unit 112 decodes the core stream, generating a decoded audio signal that is a lossy PCM signal. Meanwhile, the lossless-enhance decoder unit 113 decodes the enhanced stream to generate a residual signal. The adder 114 adds the residual signal to the lossy audio signal on the same time axis to generate a decoded audio signal that is a lossless PCM signal. This decoded audio signal is output from the audio-data decoding apparatus 110.
FIG. 3 schematically shows a configuration that the lossy-core encoder unit 101 may have in the audio-data encoding apparatus 100. As shown in FIG. 3, the lossy-core encoder unit 101 may include a band division filter 121, a sine-wave-signal extracting unit 122, a time-frequency transform unit 123, a bit allocation unit 124, and a multiplexer unit 125.
In the lossy-core encoder unit 101, the band division filter 121 divides an input audio signal into a plurality of frequency bands. The sine-wave-signal extracting unit 122 extracts sine-wave signals from the time signals of the frequency-bands and supplies parameters for constituting the sine-wave signals to the multiplexer unit 125. The time-frequency transform unit 123 performs modified discrete cosine transform (MDCT) on the time signals of the respective frequency bands, from which sine waves have been extracted. The unit 123 therefore converts these time signals to spectral signals of the respective frequency bands. The bit allocation unit 124 allocates bits to the spectral signals to generate quantized spectral signals. The multiplexer unit 125 combines the parameters for constituting the sine-wave signals and the quantized spectral signals to generate a core stream.
FIG. 4 schematically shows a configuration that the lossy-core decoder unit 102 may have in the audio-data encoding apparatus 100 described above. Note that the lossy-core decoder unit 112 provided in the audio-data decoding apparatus 110 may have the same configuration as the lossy-core decoder unit 102. As shown in FIG. 4, the lossy-core decoder unit 102 includes a demultiplexer unit 131, a sine-wave-signal reconstructing unit 132, a spectral-signal reconstructing unit 133, a frequency-time converting unit 134, a gain control unit 135, a sine-wave-signal adding unit 136, and a band-synthesizing filter 137.
In the lossy-core decoder unit 102, the demultiplexer unit 131 receives the core stream and divides the stream into parameters for constituting the sine-wave signals and quantized spectral signals. The sine-wave-signal reconstructing unit 132 reconstructs sine-wave signals from the parameters for constituting the sine-wave signals. The spectral-signal reconstructing unit 133 decodes the quantized spectral signals to generate spectral signals of frequency bands. The frequency-time transform unit 134 performs inverse MDCT (IMDCT) on the spectral signals, converting these signals to time signals of the frequency bands. The gain control unit 135 adjusts the gain of each time signal. The sine-wave-signal adding unit 136 adds a sine-wave signal to the time signal that has been adjusted in gain. The band-synthesizing filter 137 performs band synthesis on the time signals of frequency bands to generate a decoded lossy audio signal.